Dynamic Temperature Measurement

ABSTRACT

An arrangement for accurately measuring the frequency of an RF signal is disclosed. The arrangement includes a coaxial delay line carrying the RF signal and a means for directly measuring the resistance of the coaxial delay line and which resistance measurement is directly correlatable to temperature changes of the coaxial delay line. The arrangement, in response to the changes in the temperature of the coaxial delay line, provides appropriate signals to compensate for the temperature changes in order to prevent any temperature changes from degrading the accuracy of the frequency measured by the arrangement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.08/786,127, filed Jan. 17, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a frequency discriminator thatcooperates with an arrangement for measuring the frequency of an appliedRF signal. More particularly, the present invention relates to afrequency discriminator cooperating with an arrangement thatsubstantially eliminates frequency measurement delay errors typicallycaused by temperature variations in a delay line used to measure thefrequency of the applied signal. Specifically, the present inventionrelates to an arrangement that measures the resistance of the delay linecarrying the RF signal being measured and uses the measured resistanceto compensate for the temperature variations of the delay line thatwould otherwise degrade the accuracy of the frequency measurement.

2. Description of the Prior Art

RF radio receivers that measures the frequency of an applied RF signalcommonly employ a frequency discriminator. These RF radio receivers areusually characterized as digital frequency discriminators (DFD), orinstantaneous frequency measurement (IFM) receivers, both known in theart. These RF radio receivers employed for RF frequency measurementscommonly use one or more correlators also known in the art. The RF radioreceivers that utilize correlators serve well their intended purpose,but do suffer accuracy degradation due to temperature variations andsuch radio receivers may be further described with reference to FIGS.1-6 illustrated herein.

FIG. 1 illustrates a prior art arrangement 10 of a RF radio receivercomprising a frequency discriminator 12 that receives a RF signal 14having a radian frequency, ω, and delivers an output signal 16representative of a vernier or coarse measurement of the frequencycharacteristic of the applied RF signal 14.

The RF signal 14 is also applied to a correlator 18. The correlator 18directs the RF signal into two paths, the first path being a referencepath and the second path being a delay path provided by a delay device20 whose characteristic provides for a path length difference betweenthe delay and reference paths indicated by the parameter τ and measuredin seconds. As is known in the art, the correlator 18 operates in amanner analogous to performing mathematical manipulations so as toproduce one or more product signals comprising the RF signal delayed bythe path provided by the delay device 20 times the RF signal that isdirected down the reference path and does not encounter the delay device20. These products are illustrated in FIG. 1 as signals 22 and 24, eachof which contain the quantities ω and τ. The signals 22 and 24respectively representative of the phase shift encountered by the RFsignal 14 directed down the reference path, and the phase shiftencountered by the RF signal 14 directed down the delay path, moreparticularly, encountered by the delay device 20. The product signals 22and 24 are provided as output voltages from the correlator 18 andrespectively are representative of and proportional to the sine andcosine of the phase angle difference of the RF signal 14 as measuredbetween the delay and reference paths of the correlator 18. Thecorrelator 18 provides at least one of the product signals 22 and 24,but it is preferred that both product signals 22 and 24 be provided. Theproduct signals 22 and 24 may be respectively represented as sin ωτ andcos ωτ, as shown in FIG. 1.

The delay line 20 may be implemented by a coaxial cable, printed delayline or some other delay device, and by choosing the propercharacteristic of the delay line path (as known in the art) a desiredinterrelationship between the RF input frequency of the RF signal 14 toa video output (to be described with reference to FIG. 9) can beachieved. Moreover, improved measurement resolutions and accuracy of theRF frequency may be provided (as known in the art) by the use ofmultiple correlators 18, to be further described hereinafter withreference to FIG. 5, arranged to perform mathematical manipulationswhich correspondingly provide for more accurate product output signals22 and 24. However, in spite of further accuracy improvements for theproduct signals 22 and 24, the use of the delay line 20 degrades theaccuracy of the RF frequency measurement.

The accuracy of the RF frequency measurement signal is a direct functionof the accuracy of the RF delay line 20. The delay line 20 typicallyexhibits a repeatable error characteristic which is temperaturedependent. To eliminate this temperature dependent error, a variety ofapproaches have been employed to compensate for changes in temperatureand its attendant degradation of the accuracy of the frequencymeasurement and one such approach may be described with reference toFIG. 2.

FIG. 2 illustrates a prior art arrangement 26 which is quite similar tothat of FIG. 1 with the exception that an oven 28 has been added theretoand in which is arranged the delay device 20. The delay device 20 is notsubjected to any accuracy degrading temperature variations, because ofthe constant temperature provided by the oven 28. The arrangement 26having the oven 28 serves well its intended purpose, but has theattended drawbacks of power consumption, the need of warm-up time toallow the temperature of the oven to stabilize before the operation ofcircuit arrangement 26 is initiated, and also the disadvantage of theunreliability of the oven 28. Furthermore, as previously mentioned, whenemploying multiple correlators, such as those of FIG. 5, to be furtherdescribed hereinafter, placed into a parallel array and arranged in afixed ratio, relative to each other and having typical values of 2:1;3:1 or 4:1, all of the temperatures of all of the RF delay lines areusually not stabilized. When such is the case, the characteristics ofthe RF delay lines will differ between the oven stabilized lines andthose that are not. This difference is accentuated over temperature, andcan lead to temperature tracking errors and large errors in the outputfrequency data measurements. An arrangement that does not suffer theinherent oven drawbacks may be described with reference to FIG. 3.

FIG. 3 illustrates a prior art arrangement 30 which is similar to thatof FIG. 1 except that the conventional delay line 20 of FIG. 1 isreplaced with a relatively large (compared to delay line 20) coaxialdelay line 32, known in the art, having a low temperature coefficient.The arrangement 30 substantially corrects the temperature variationdegradation of the accuracy of measuring frequency without suffering thedisadvantages of the oven 28 of FIG. 2, but it does suffer thedisadvantages of a higher cost (relative to delay line 20 ) and alsopackaging problems because of its relatively large size. Furthermore,because the special low temperature coefficient coaxial cable delaylines 32 are, in general, factory assembled, it causes field maintenanceproblems. An arrangement that does not suffer the disadvantages ofeither FIGS. 1, 2 or 3 may be further described with reference to FIG.4.

FIG. 4 illustrates a prior art arrangement 34 which has many of thefeatures of FIGS. 1-3 including a frequency discriminator 12 thatreceives the RF input signal 14 and provides an analog output signalrepresentative of the frequency of the RF signal which, for arrangement34, is routed to an analog to digital converter 40. The analog todigital converter 40 provides a first digital signal 16A representativeof data that corresponds to the coarse frequency of the RF signal 14which is routed to a programmable memory device 42. The programmablememory device 42, to be further described hereinafter with reference toFIG. 6, may be a conventional processor that is responsive to a programthat causes the programmable memory device 42 to operate in apredetermined manner to combine digital signals. FIG. 4 furtherillustrates the arrangement 34 as having an analog to digital converter44 which receives the product signals 22 and 24 and converts them intorespective digital data which are routed to the programmable memorydevice 42. The τ quantity of the product signals 22 and 24 sine ωτ andcosine ωτ, respectively, is produced by a delay line 46.

The delay line 46 may be either a coaxial cable, a printed delay line,or another equivalent device, all known in the art, which cooperateswith a temperature sensitive resistor 48 that is thermally coupled tothe delay line 46 by means of a thermally conductive compound 50, allknown in the art. The temperature sensitive resistor 48 has its firstend connected to a first end of resistor RT1 which has its second endconnected to ground and, further, the temperature sensitive resistor 48has its second end connected to a first end of resistor RT2 which hasits second end connected to a voltage reference 53. The temperaturesensitive resistor 48 develops a voltage thereacross that is applied toan operational amplifier 52, by means of signal paths 54 and 56. Theoperational amplifier 52 amplifies the received signal and develops anoutput voltage, sometimes referred to as a video temperature voltage,which is applied to an analog to digital converter 52A. The analog todigital converter 52A provides a second digital signal representative ofthe temperature being sensed by the delay line 46.

The programmable memory device 42 serves as a combiner to combine thedata generated by the analog to digital converter 52A, serving astemperature dependent additive factor, with the data contained in signal16A (coarse frequency data) so as to provide frequency and temperaturedependent data to produce an accurate measurement signal 60 of thefrequency of the RF signal 14 that is compensated for temperatureerrors. The arrangement 34 of FIG. 4 may be further described withreference to FIGS. 5 and 6 associated with digital frequencydiscriminators (DFD) and with FIG. 5 illustrating a general arrangement36A and FIG. 6 illustrating an arrangement 38B showing some of theessential details of FIG. 5.

FIG. 5 illustrates the arrangement 36A as comprising seven correlators,indicated as 18A, 18B, . . . 18F and 18G, with correlator 18G having theparallel temperature sensing resistor 48 indirectly coupled to thelongest RF device line, that is, the delay line of the correlator 18G.The correlators 18A, 18B, 18F and 18G receive their RF signal 14 from apower divider 38 that receives the incoming RF signal 14. Each of thecorrelators 18A, 18B, . . . 18F and 18G produces the product signals 22and 24 (sin ωτ and cos ωτ) which are routed to the arrangement 36B ofFIG. 6.

FIG. 6 illustrates the programmable memory device 42 as comprised ofelements 42A (TTL comparators and latches), 42B (error correction prom),42C (temperature correction prom), 42D (output data latch), and 42E(phase split prom). FIG. 6 further illustrates amplifier 44A and 44Bthat respectively receive the sin ωτ and cos ωτ quantities of theseventh correlator 18G.

The differential sine (sin ωτ) and cosine (cos ωτ) video outputs fromcorrelators 18A through 18F are provided to TTL comparators and latches42A. This provides a 12-bit address input to the error correction prom42B. In parallel, the sine (sin ωτ) and cosine (cos ωτ) video from theseventh (and longest delay) correlator 18G is amplified by videoamplifiers 44A and 44B and are provided to ADC (Analog to DigitalConverter) 44. The Most Significant Bit (MSB) of the digitized sine andcosine data (MSB S7 and MSB C7 ) of the ADC 44 respectively present onsignal paths 44C and 44D are provided to the error correction prom 42B.This, when combined with the 12-bit data from correlators 18A through18F, provides a 14-bit address to the error correction prom 42B. Theerror correction prom 42B is programmed with a mathematically generatederror correction algorithm, known in the art, which produces an 8-biterror corrected raw coarse frequency data word on signal path 42F. Thisis the 8 MSB of frequency data prior to temperature correction.

With reference to the digitized sine and cosine data of correlator 18G,these two 6-bit data words are provided to the phase split prom 42Ewhich, based on an analysis of the digitized sine and cosine video data,provides three outputs. More particularly, by using an arc-tangentalgorithm, it provides the 5 LSB (Least Significant Bits) of a rawfrequency data word indicated as signal path 42G. These 5-bits areattached to the 8-bit raw coarse frequency data word of signal path 42Fto form the 13-bit raw frequency data word. In addition, the phase splitprom 42E also generates a coherent threshold and data valid, which is anestimate of the utility of the data and which is indicated as signalpath 42H.

In parallel, the temperature video, from the temperature sensor attachedto the RF delay line, is digitized, forming a 5-bit temperature dataword indicated as signal path 42I. This data is combined by thetemperature correction prom 42C with the 13-bit raw frequency data wordto generate a±correction which is added to the 13-bit raw frequency dataword, to produce a 12-bit corrected frequency data word on signal path42J which is routed to the output data latch the output of which issignal 60 of FIG. 4. The reason for the reduction from 13-bits to12-bits is that it is desired to correct for temperature at half thefrequency resolution so as to avoid dealing with the temperatureboundaries at the digital output.

It should be noted that the actual technique for combining thetemperature data with the raw frequency data is to form a 13-bit addressfor a temperature correction prom 42C, using the 8 MSB of frequency dataand the 5-bits of temperature data. This prom 42C output is a±sign bit,plus 7-bit correction data, with the LSB of this data at half the outputfrequency resolution. This 7-bit data word is added to (or subtractedfrom, depending on the±sign bit) the 13-bit raw frequency data word in abinary full adder circuit. At the output of the adder circuit, the LSBis ignored, to produce the 12-bit frequency data output on signal path42J.

While the arrangement 34 of FIG. 4 provides for accurate measured RFfrequency data that serves an intended purpose, especially for statictemperatures, there is a drawback with the arrangement 34 with regard todynamic temperature changes. More particularly, in the arrangement 34there is a time lag for sensing the changes in the temperature that thetemperature resistor 48 is subjected to relative to the temperature ofthe delay device 46. This time lag is created by imperfections of thecompound 50 coupling the temperature sensitive resistor 48 to the delaydevice 46. This time lag produces an error associated with thetemperature sensitive resistor 48 and the delay device 46, especially atdifferent temperatures. This time lag effect is particularly noticeablewhen the RF radio receivers of FIG. 4 are employed in high performanceaircraft, where the temperature of the environment in which the RF radioreceivers need to operate may change more than 50° C. in less than 5minutes.

It is desired that an arrangement for measuring the frequency of anapplied RF signal be provided that is substantially free fromtemperatures variations that might otherwise degrade the accuracy of thefrequency measurement.

Accordingly, it is a primary object of the present invention to providefor an arrangement for measuring the frequency of an applied RF signalwhose accuracy is not degraded by changes in temperature. Moreparticularly, it is of prime importance to the present invention todirectly measure the temperature of the delay line carrying the RFsignal without the use of indirect device such as sensing resistorconnected across the delay line of the correlator and use thatmeasurement to directly compensate for any temperature changes thatmight otherwise degrade the accuracy of the measurement of the frequencyof the applied RF signal.

It is a further object of the present invention to provide a correlatorand associated circuitry used in frequency measurements that compensatefor temperature variations.

Further still, it is an object of the present invention to providecircuitry used in the reference or delay line path of a correlator thatcompensates for temperatures variations therein.

Still further, it is an object of the present invention to provide foran arrangement that receives an applied RF signal, digitizes the appliedsignal, and provides a digitized output signal representative of anaccurate frequency measurement of the applied RF signal.

It is another object of the present invention to provide for anarrangement that measures the resistance of the delay path of thecorrelator and uses that resistance measurement to provide for a signalthat is used to compensate for temperature variations.

In addition, it is an object of the present invention to provide for acoaxial delay line in which the resistance of the center conductorand/or the outer shield is measured to provide for a signal thatcompensates for the temperature variations to which a correlator issubjected.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus for accuratelymeasuring the frequency of an applied RF signal by measuring theresistance of a delay path through which the RF signal flows and thenutilizing the resistance measurement to compensate for any temperaturevariations that the arrangement may be subjected to that might otherwisecause accuracy degradation to the frequency measurement.

The apparatus comprises a frequency discriminator, first and secondanalog to digital converters, a correlator, means for measuringresistance, detecting means, and a combiner. The frequency discriminatorreceives an applied RF signal and provides an analog output signalrepresentative of the frequency of the RF signal. The first analog todigital converter receives the analog signal and provides a firstdigital representation thereof. The correlator receives and directs theRF signal into a delay path and a reference path. The correlatorproduces an output voltage representative of and proportionate to atleast one of the sine and cosine of the phase angle difference of the RFsignal as measured between the delay and reference paths. The measuringmeans is connected to one of the delay and reference paths and measuresthe corresponding resistance of one of the delay and reference paths anddevelops an output signal representative of the measured resistance. Thedetecting means receives the output signal of the measuring means,detects changes thereto, and develops an output signal representative ofthe changes thereof. The second analog to digital converter receives theoutput of the detecting means and provides a second digital signalrepresentative thereof. The combiner receives the first and seconddigital signals and combines them into a composite signal representativeof the frequency of the applied RF signal.

The attainment of the foregoing and related objects, advantages andfeatures of the invention should be more readily apparent to thoseskilled in the art, after the review of the following detaileddescription of the invention taken in conjunction with the drawings inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art arrangement for measuring thefrequency of an applied RF signal, but suffering from accuracydegradation due to the inability to compensate for temperaturevariations.

FIG. 2 is a block diagram of a prior art arrangement for measuring thefrequency of an applied RF signal and utilizing an oven to compensatefor temperature variations that might otherwise cause accuracydegradations.

FIG. 3 is a block diagram of a prior art arrangement for measuringfrequency of an applied RF signal that utilizes a low temperaturecoefficient delay line to substantially prevent temperature variationsfrom degrading the frequency measurement.

FIG. 4 is a block diagram of a prior art arrangement for measuring thefrequency of an applied RF signal that utilizes a temperature sensitiveresistor to serve as a sensor for indirectly sensing and compensatingfor temperature variations that might otherwise degrade the accuracy ofthe frequency measurement.

FIG. 5 is a block diagram of a prior art arrangement of a plurality ofcorrelators used for frequency measurement.

FIG. 6 is an arrangement showing further details of the prior art blockdiagram of FIG. 5.

FIG. 7 is a block diagram of an arrangement of the present inventionthat measures the frequency of an applied RF signal and utilizes acircuit that directly measures the resistance of the delay path of thecorrelator to directly compensate for temperature variations that mightotherwise degrade the accuracy of its measurements.

FIG. 8 is similar to the arrangement of FIG. 7, but has the measuringmeans directly attached to the reference path of the correlator.

FIG. 9 illustrates further details of the arrangement of the presentinvention of FIG. 7.

FIG. 10 illustrates a detector used in the circuit arrangement of FIG.9.

FIG. 11 illustrates the results of testing performed on the circuitarrangement of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, wherein the same reference numbersindicate the same elements throughout, there is shown in FIG. 7 acircuit arrangement 62 that accurately measures the frequency of theapplied RF signal 14, previously described with reference to FIGS. 1-6.The circuit arrangement 62 of FIG. 7 is similar to the circuitarrangement 34 of FIG. 4, with the exception of the associated circuitry(to be further described hereinafter) used in cooperation with eitherthe reference or delay path of the correlator 18.

In general, the circuit arrangement 62 directly measures the resistancein the delay path that carries the RF signal 14, converts thisresistance to a voltage (AC or DC), digitizes this voltage (AC or DC)into digital data, and uses the digital data to provide a compensationquantity for any variations in the RF signal 14 in the delay path thatmay have been caused by temperature changes. Although it is preferredthat the resistance of the delay line associated with the correlator 18be directly measured, the practice of this invention is also applicableto the direct measurement of resistance of the reference path of thecorrelator 18.

The elements of circuit arrangement 62 that have not been alreadydescribed with reference to FIGS. 1-6 are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        REFERENCE NO.  ELEMENT                                                        ______________________________________                                        C1             BLOCKING CAPACITOR                                             C2             BLOCKING CAPACITOR                                             64             COAXIAL DELAY LINE                                             RN1            FIRST RESISTOR NETWORK                                         RN2            SECOND RESISTOR NETWORK                                        66             CONSTANT VOLTAGE SOURCE                                        68             OPERATIONAL AMPLIFIER                                          ______________________________________                                    

A measuring means of the arrangement 62 comprising the first resistornetwork RN1, the second resistor network RN2, first and second blockingcapacitors C1 and C2, and the voltage source 66 having first and secondterminals. The measuring means is connected to the coaxial delay line64. The values of C1 and C2 vary with the frequency band at which thisinvention is employed, but each of these blocking capacitors C1 and C2is selected to have a value so as to provide extremely high DCresistance preferably in excess of 1 megohm and a capacitive reactancepreferably less than 5 ohms. The coaxial delay line 64 is preferably acoaxial type with a semi-rigid center conductor having a typicaldiameter of 0.047 inches. The coaxial delay line 64 has a repeatablevariation in its electrical delay length as a function of temperature.The center conductor of the coaxial delay line 64 also has a repeatablevariation of resistance as a function of temperature. The principles ofthe present invention utilize these repeatable variations to compensatefor temperature changes in a manner as to be described.

A detecting means of the circuit arrangement 62 comprises theoperational amplifier 68 that has an operative relationship with boththe first resistor network RN1 and the second resistor network RN2 whichare respectively connected to an entrance portion 70 and exit portion 72of the coaxial delay line 64. The first and second networks RN1 and RN2are preferably connected to the center conductor of the coaxial delayline 64. The second resistor network RN2 receives excitation from theconstant voltage source 66 via branch 74B which is connected to the mainconductor branch 74 connected to the constant voltage source 66.

The constant voltage source 66 has two terminals one of which isconnected to branch 74 and the other of which is preferably connected tothe ground potential, as shown in FIG. 7. The constant voltage source 66may supply either AC or DC excitation with a typical voltage value ofabout 5V.

In operation, the constant voltage source 66 provides for the flow of asmall current, typically in the range from 5 to 10 milliamperes, throughresistor network RN2, the coaxial delay line 64, the resistor networkRN1, and then to ground. The coaxial delay line 64 has a finite, butsmall resistance. The current passing through this resistance causes avoltage drop to appear between the entrance portion 70 and exit portion72 of the coaxial delay line 64. The voltage between the entrance 70 andexit 72 is applied to the negative (-) and positive (+) inputs ofoperational amplifier 68 via signal paths 78 and 80 respectively. Theoperational amplifier 68 develops and supplies its output to the analogto digital converter 58 which, in turn, converts it to the digitalrepresentation 82 serving as temperature compensated data which iscombined, by the programmable memory device 42, with the coarsefrequency data 16A to generate frequency correction data 84representative of frequency of the RF signal 14 and compensated for anyerrors that might otherwise be caused by temperature changes to thecoaxial delay line 64.

The programmable memory device 42, previously described in detail withreference to FIG. 6, may be pre-programmed in a manner known in the artso that increases in the resistance of the coaxial delay line 64,manifested as corresponding increases in the temperature of the coaxialdelay line 64, are used to decrease the measured frequency of the RFsignal 14 as represented by the data 84. Other techniques may be used,and even devices other than a programmable memory device 42 may be usedso long as the data 84 representing the frequency of the applied RFsignal 14 is compensated for the temperature changes be sensed by thecoaxial delay line 64.

Furthermore, the practice of the present invention is not limited tobeing based on the measurement of delay line 64, but rather may bepracticed for any delay line, or any number of delay lines arranged inany predetermined array, or on any RF line such as the reference line ofthe correlator 18 as shown for arrangement 62A of FIG. 8.

The arrangement 62 A of FIG. 8 is the same as arrangement 62 of FIG. 7except that the resistive networks RN1 and RN2, the voltage source 66,the operational amplifier 68 and the analog/digital converter 58 aredirectly applied to the reference line (shown in phantom) of thecorrelator 18. The interconnection of these elements of the arrangement62A of FIG. 8 are the same as described for the arrangement 62 of FIG.7.

In general, it is preferred that the application of the temperaturesensing of the present invention be directly applied to the longest RFdelay line only because this line, being the longest, has the highestresistance, and thereby is the easiest device to directly measure thechange over temperature. The essential aspect of the present inventionis that of directly measuring the actual delay line temperature; itmatters not which delay line, as they are all assumed to be at the sametemperature, even dynamically. It should be noted that unlike prior artdevices, such as sensing resistors indirectly connected to a line to bemeasured, the present invention directly measures the line and, thus, isnot burdened with the errors associated with the indirect device, suchas those of the sensing resistor. The operation of the arrangements ofFIGS. 7 and 8 may be further described with reference to FIG. 9 thatgives further details related to the arrangement 62 although thesedetails are equally applicable to arrangement 62A of FIG. 8.

FIG. 9 illustrates the first and second resistor networks RN1 and RN2and the operational amplifier 68 and its associated components groupedinto a circuit arrangement 86. As a comparison between FIGS. 7 and 9reveals only the blocking capacitor C1, which is located on one side ofthe coaxial delay line 64, is illustrated in FIG. 9 and this is becausethe blocking capacitor C2 is not needed because of the grounding actionof an inductor LF within detectors 102, 104, 106 and 108, to bedescribed hereinafter with reference to FIG. 10, completes the DCcircuit on the other side of the coaxial delay 64. The first resistornetwork RN1 comprises a potentiometer R1 having a typical value of 10kΩ, a resistor R2 having a typical value of 2 kΩ, and a precisionresistor R3 having a typical value of 1.5 Ω, whereas the second resistornetwork RN2 comprises a single resistor having a typical value of 1 kΩ.The operational amplifier 68 has a resistor R4 and a capacitor C3arranged as shown to provide feedback between the output and the inputstages of the operational amplifier 68 and having respective values of300Ω and 0.1 microfarads.

The resistive network RN2 has one end connected to a first end of aninductor L1 which, in turn, has its second end connected to the entranceportion 70 of the coaxial delay line 64 and also to one end of ablocking capacitor C1 which, in turn, has its second end connected to apower coupler 88 which is actually a part of the correlator 18. Thepower coupler 88 has first and second ends 88A and 88B, with the lineattached to end 88B serving as the reference line that is illustrated inphantom in FIG. 8. The inductor L1 may be a piece of bent wire withextremely low DC resistance preferably less than 0.5 ohms and preferablyhaving an inductive reactance in excess of 500 ohms.

The arrangement 62 of FIG. 9 illustrates the correlator 18 as being amicrowave correlator (known in the art) assembly made available fromWide Band Systems, Inc., of Franklin, N.J. and having first, second,third and fourth couplers 92, 94, 96 and 98 respectively. The firstcoupler 92 is arranged for receiving the RF signal 14 (not shown) afterit has passed through the delay path provided by the coaxial delaydevice 64. The first coupler 92 has first and second ends 92A and 92B.The second coupler 94 has first and second ends 94A and 94B with thefirst end 94A arranged for receiving the signal present at the first end92A of the first coupler 92.

The third coupler 96 has first and second ends 96A and 96B, with thefirst end 96A arranged for receiving the signal present at the secondend 92B of the first coupler 92. The fourth coupler 98 has first andsecond ends 98A and 98B, with the first end 98A arranged for receivingthe signal present at the second end 94B of the second coupler 94. Thesecond end 98B of the fourth coupler 98 is arranged for receiving thesignal that is present at the second end 96B of the third coupler 96 andis also connected to a terminating resistor 100, having a typical valueof 75Ω.

The second end 94B and the first end 94A both of the coupler arerespectively connected to detectors 102 and 104 and, similarly, thesecond end 96B and the first end 96A both of the third coupler 96 arerespectively connected to detectors 106 and 108. All of the detectors102, 104, 106 and 108 comprise an arrangement of a diode, an inductorand a capacitor that may be further described with reference to FIG. 10.

FIG. 10 illustrates the detectors 102, 104, 106 and 108 as comprising adiode D1 having an anode and a cathode with the anode and cathoderespectively serving as the input 110 and the output 112 connections ofthe detector. The detectors 102, 104, 106 and 108 further comprise aninductor LF with the first end connected to the anode of the diode D1and the second end connected to the ground potential which is common tothe ground of the constant voltage source 66. The detectors 102, 104,106 and 108 still further comprise a capacitor CF with first and secondends, with the first end connected to the cathode of the diode D1 andthe second end connected to the ground potential. The detectors 102,104, 106 and 108 are commercially available from ACC, Inc., and inoperation the inductors LF provide a DC path to ground with an inductivereactance greater than 500 ohms. The capacitors CF complete the RFcircuit to ground, blocking DC to ground, and each has a capacitivereactance typically less than 5 ohms at the RF operating frequency atwhich this invention is practiced.

In operation, and with simultaneous reference to FIGS. 9 and 10, theoperational amplifier 68 is arranged, as is shown in FIG. 9, to serve asa common DC resistance bridge which compares the resistance of thecoaxial delay line 64 to that of the small precision resistor R3. Theresistors R1 and R2 provide a small current to the precision resistorR3, whereas the second resistor network RN2 provides a similar smallcurrent through the choke L1, into the entrance portion 70 of thecoaxial delay line 64, through the coaxial delay line 64, out to theexit portion 72 of the coaxial delay line 64, and then to the coupler 92of the correlator 18. The correlator 18 is arranged so that the couplers92, 94, 96 and 98 drive the detector modules 102, 104, 106 and 108. InFIG. 10 it should be noted that each of the detector modules 102, 104,106 and 108 has an inductor LF connected to ground at the diode D1input. The current originating through the second resistor network RN2flows through the coaxial delay line 64 then to ground through thedetectors 102, 104, 106 and 108. The output of detector 102 is theproduct signal 22 (cosine ωτ), whereas the output of the detector 104 isproduct signal 22' which serves as a differential video signal. Thedetectors 102 and 104 produce a differential cosine output, and,similarly, the detectors 106 and 108 produce a differential sine output.The differential video output is used to reduce the sensitivity of videocircuits to extraneous noise pick-up. The output of the detector 106 isthe product signal 24 (cosine ωτ), whereas the output of detector 108 is24' which serves as a video signal. The product signals 22 and 24 aswell as the video signals 22' and 24', are routed to the analog todigital converter 44 which provides a digital representation thereofthat is routed to the programmable memory device 42.

In the overall operation of the circuit arrangement of FIG. 9, as theresistance of the coaxial delay line 64 varies with temperature, the DCvoltage at the entrance 70 of the coaxial delay line 64 changes which,in turn, changes the voltage at the second end of the second resistornetwork RN2 which is also connected to the positive (+) input of theoperational amplifier 68. The operational amplifier 68 amplifies thischange and provides a corresponding increased voltage to the analog todigital converter 58 which, in turn, provides a corresponding increaseddigital representation of the temperature related quantity.

The RF signal 14 applied to the power divider 88 is unaffected by thechanges sensed by the operational amplifier 68. More particularly, theRF signal 14 is passed through the DC blocking capacitor C1 and isisolated from the operational amplifier 68 by the choke L1. The RFsignal 14 then continues (along with the small DC current) to thecoaxial delay line 64 and is processed normally by the correlator 18.

It should now be appreciated that the practice of the present inventionprovides for an arrangement for accurately measuring the frequency of anapplied RF signal and which accuracy is unaffected by the temperaturechanges that may affect the RF delay device.

It should be further appreciated that unlike prior art devices thatutilizes devices, such as sensing resistors coupled to a delay line toindirectly measure temperature changes, a critical feature of thepresent invention is that it directly measures the temperature of thedelay line without the need of sensing resistors and provides anaccurate measurement thereof so that the frequency of the applied RFsignal can correspondingly be accurately measured.

It should be further appreciated that the arrangement of FIGS. 7, 8 and9 of the present invention accepts either an AC or DC excitationsupplied by the constant voltage source 66 and operates correctly foreach type of excitation.

Although the present invention describes the measurement of theresistance of the center conductor of the coaxial delay line 64, itshould be recognized that the resistance of the outer shield or thecombination of the resistance of the outer shield and that of the centerconductor of the coaxial delay line 64 may be used. If the outer shieldof the coaxial delay line 64 is used then means need to be provided sothat the inductor L1 is connected to the outer shield of the coaxialdelay line 64.

RESULTS YIELDED FROM THE PRACTICE OF THE PRESENT INVENTION

In the practice of the present invention tests were performed using thearrangement 62 of FIGS. 7, 9 and 10, and the results of which areillustrated in FIG. 11 for a response plot 114. FIG. 11 has a Y axisindicated in terms of the output voltage of the operational amplifier68, and a X axis indicated in terms of temperature, and given in degreescentigrade (C). The output voltage indicated in the Y axis isrepresentative of the difference in the voltage detected by theoperational amplifier 68 as compared to the circuit arrangement of FIG.2 that uses in an oven 28 to provide a constant temperature for thecoaxial delay line. More particularly, the oven 28 provides a constanttemperature for a coaxial delay line 64, whereas the response plot 114of FIG. 11 represents the changes in the voltage (Y axis) sensed by theoperational amplifier 68 as a coaxial delay line 64 experiences thetemperatures and changes thereto (X axis).

The testing consisted of run one (plot 116) and run two (plot 118) whichwere made in two different temperature directions, with run one (plot116) starting from cold and going to hot temperatures, and run two (plot118) starting from hot and going to cold temperatures. Plot 116 isindicated with solid marker blocks, whereas plot 118 is partiallyindicated with marker blocks shown in phantom.

From FIG. 11 it is seen that run two, plot 118, had lower voltage valuesat the low temperature quantities compared to those of plot 116, whereasat high temperature values plot 118 has values that exceed those of plot116. It is also seen from FIG. 11 that both plots 116 and 118 aresubstantially linear. It may be further seen from FIG. 11 that there isa slight gap between plots 116 and 118 at cold temperatures which is dueto the error between the measured air temperature in the oven (28) andthe actual coaxial delay line 64 temperature, which, of course,emphasize the difficulties encountered in measuring the actual delaydevice temperatures.

It should now be appreciated that the practice of the present inventionprovides for an arrangement that accurately measures the frequency of anapplied RF signal in the microwave frequency range and provides suchaccuracy in spite of any temperature variations that the delay line ofthe correlator may experience.

While the invention has been described with reference to specificembodiments, this description is illustrated and is not to be construedas limiting the scope of the invention. Various modifications andchanges may occur to those skilled in the art without departing from thespirit and scope of the invention as defined by the appended claims.

What we claim is:
 1. A correlator comprising:a) means for receiving anddirecting an RF signal into both a delay path and a reference path anddeveloping an output voltage representative of and proportional to atleast one of the sine and cosine of the phase difference of the RFsignal as measured between the delay and reference paths; b) resistancemeasuring means directly connected to one of the delay and referencepaths for directly measuring the resistance of the corresponding delayand reference paths and developing an output signal representative ofsaid directly measured resistance; and c) detecting means receiving theoutput signal of said direct resistance measuring means for detectingchanges thereto and developing an output signal representative of thechanges thereof.
 2. The arrangement according to claim 1, wherein saidresistance measuring means comprises:a) a two terminal voltage source;b) a coaxial delay line comprising a center conductor for carrying saidRF signal and having entrance and exit portions with means forrespectively receiving and conveying said RF signal; c) a first resistornetwork having first and second ends with the first end connected tosaid entrance portion and the second end connected to one terminal ofsaid two terminal voltage source; and d) a second resistor networkhaving first and second ends with the first end connected to said exitportion of said coaxial line and the second end connected to the otherterminal of said voltage source.
 3. The arrangement according to claim2, wherein said detecting means is directly connected to receive saidoutput signal representative of said directly measured resistance, saiddetecting means comprising:an amplifier having positive (+) and negative(-) terminal inputs respectively connected to the first ends of thefirst and second resistor networks and having means for developing saidoutput signal of said detecting means representative of the differencebetween the voltage between its positive (+) and negative (-) terminals.4. A method of measuring the temperature of a correlator having delayand reference paths so that temperature changes to the correlator doesnot degrade the measurement accuracy of the correlator, said methodcomprising the steps of;(a) connecting resistance measuring meansdirectly to one of said reference and delay paths for directly measuringthe resistance of said one of said reference and delay paths anddeveloping an output signal representative of said directly measuredresistance; and (b) directly connecting detecting means to receive theoutput signal of said direct resistance measuring means for detectingchanges thereto and developing an output signal representative of thechanges thereof.
 5. The method according to claim 4, wherein saidresistance measuring means comprises:a) a two terminal voltage source;b) a coaxial delay line comprising a center conductor for carrying a RFsignal applied to said correlator and having entrance and exit portionswith means for respectively receiving and conveying said RF signal; c) afirst resistor network having first and second ends with the first endconnected to said entrance portion and the second end connected to oneterminal of said two terminal voltage source; and d) a second resistornetwork having first and second ends with the first end connected tosaid exit portion and the second end connected to the other terminal ofsaid voltage source.
 6. The method according to claim 5, wherein saiddetecting means comprises:an amplifier having positive (+) and negative(-) terminal inputs respectively connected to the first ends of thefirst and second resistor networks and having means for developing saidoutput signal of said detecting means representative of the differencebetween the voltage between its positive (+) and negative (-) terminals.